Electrical conductor with improved hydrocarbon insulator



June 30, 1942. w. M. SMITH ET AL 2,288,373

ELECTRICAL CONDUCTOR WITH IMPROVED HYDROCARBON INSULATORS Filed Feb. 27, 1939 F76. -Z F761 F I 0/1. ram z #ramy 41871 0404 0 SUAF/DE Patented June 30, 1942 ELECTRICAL CONDUCTOR WITH IMPROVED HYDROCARBON IN SULATOR Warren M. Smith and Carroll J. Wilson, Eliza beth, N. J., assignors to Standard Oil Development Company, a corporation of Delaware Application February 27, 1939, Serial No. 258,662

11 Claims.

This invention relates to insulated electrical conductors, and is particularly concerned with the use of an addition compound in a hydrocarbon insulating oil to form a stabilized insulator which performs more efllciently under electrical stress in practical application to high voltage cable and capacitor apparatus. The addition com-pound used is selected from a class of alkyl hydroxy aryl sulfides which can be incorporated into an insulating oil in adequate amounts, due to their high solubility in saturated hydrocarbons, to be effective in markedly suppressing gas evolution caused by the action of electric discharges on such hydrocarbons.

Insulating oils principally employed in electrical transformers and cables are well refined viscous hydrocarbon oils, which in spite of their high oxidation stability obtained through careful refining, tend to deteriorate under the oxidation promoting action of fairly high temperatures and catalytic action by metals to which they become subjected in operation of high voltage electrical apparatus. The function of the oils is to insulate electrical conductors and to carry away heat developed in the conductors.

Although hydrocarbon dielectrics in the form of oils and waxes for use as insulators have been developed through advances in the refining art to have high resistance to oxidation and a number of anti-oxidants have been proposed to further retard oxidation of the refined hydrocarbons, other deteriorating causes have been found to be exerted on the oils in their actual application and these could not be explained on the basis of oxidation efiects experienced in the common uses of viscous hydrocarbon oils. Compounds such as aromatic amines, aromatic sulfides, nitrobenzene, phenols, etc., have been added to refined hydrocarbons in the usual manner as catalysts for inhibiting oxidation but their improvements in diminishing color and acid formation are not sufllcient to eliminate the troubles experienced with hydrocarbon dielectrics electrical apparatus.

Further harmful effects of gas formation and polymerization from-the action of gaseous electric discharge on hydrocarbons need serious attention in improving insulating oils. This deteriorating influence in electrical apparatus is fundamentally independent of oxidation because it occurs mainly at low temperatures, in the absence of oxygen, and with increased effects on the more chemically stable oils. It is regarded as originating in the ionization of gas existing in small bubbles or pockets-mainly at periods when the temperature of the insulating oil drops. A gaseous electric discharge of high velocity ions formed by this ionization vigorously bombards the surrounding hydrocarbon molecules and thereby decomposes them to cause liberation of hydrogen gas and formation of polymers. The gas liberated forms gas pockets aocentuating further ionization which may finally lead to a breal down of the insulation. The polymers form a wax-like deposit which is not so detrimental in itself, but which evidences gas evolution and is conducive to further ionization and gas evolution by retarding the flow} of the insulating liquid.

The chief purpose of this" invention is to improve insulating oils by providing addition compounds which not only inhibit oxidation, but which can be used in sufficient concentrations to materially decrease deleterious effects of electric discharges. Even though the substances efficiently serving this purpose contain combined sulfur, they have been found meritorious in avoiding corrosion of copper, corrosion acceleration in the presence of lead compounds which are inevitably formed in the apparatus, and sludge formation under conditions encountered in the apparatus. These addition compounds are best represented by di( tertiary amyl phenol) sulfide and di(tertiary amyl beta-naphthol) sulfide which belong to the class of alkyl hydroxy aryl sulfides having suitably high hydrocarbon oil solubilities. This class of compounds, in general, is characterized by molecular compositions in which aromatic nuclei, linked together through one or two sulfur atoms, contain alkyl group substituents of preferably 4 to 6 carbon atoms and hydroxy group substituents. The alkyl groups are important substituents because they influence the solubility and stability of the compound. Secondary and tertiary amyl and butyl hydrocarbon radicals are indicated as being most satisfactory alkyl group substituents. Other functional groups, such as an amino group, may also be present as substituents.

The manner in which the addition compounds are employed and the experimental technique by which they are tested for proving their effectiveness will be fully understood from the following description.

The accompanying drawing diagrammatically illustrates specific embodiments of the invention.

Fig. I is a cutaway longitudinal view of an electrical conductor insulated by a spiral wrapping of paper impregnated with a refined hydrocarbon dielectric containing a suflicient quantity of the addition compound in solution to suppress gas evolution. Fig. II shows a cross-sectional view at II-II in a cable having the construction shown in Fig. I.

Fig. III shows a cutaway longitudinal view of an oil filled cable of a type in which a free flowing insulating oil stabilized according to the present invention is maintained under pressure. Fig.

IV shows a cross-sectional view of a cable having the construction shown in Fig. III, the section being taken at III-III in Fig. III.

Referring to Fig. I and Fig. II, the metallic electrical conductor I commonly constructed of wound copper, is surrounded by a spiral wrapping of paper 2 impregnated with a stabilized hydrocarbon dielectric which in turn is encased within a lead sheath 3.

In Figs. III and IV, three electrical conductors I in the form of copper windings are wrapped by paper tape 2 impregnated with a stabilized hydrocarbon dielectric preferably of the transformer oil or light cable oil type. The conductors are encased within a lead sheath 3 from which they are centrally spaced and insulated by suitable insulating means (not shown). Loose spiral copper tubes 5 are disposed between the lead sheath interior and wrapped conductors to insure free passage of the oil and uniformity of oil pressure in space 4. A thin copper sheet 6 is lapped as a reinforcement around the paper wrapping 2 sur-,

rounding each conductor l.

Electrical conductors employed in oil filled transformers are similarly constructed of a metallic conductor wrapped with a cellulosic paper or fabric which is impregnated by surrounding transformer oil, no lead sheath or metallic casing being employed, of course. as in cable construction.

Blends of alkyl hydroxy aryl sulfides in various concentrations with a well refined electrical cable oil which by itself evolves considerable quantities of fixed gas when exposed to a high voltage electric discharge, were tested in a concentric tube ozonizer under conditions simulating those in high voltage cables. This was accomplished by having a metal electrode partially Immersed in an insulating liquid to be tested, which partly filled a glass tube surrounded by a body of salt water acting as the other electrode. A 60 cycle alternating current at 15,000 volts was applied to the electrodes. The discharges produced in a hydrogen gas space above the insulating liquid caused the liquid to foam and gas pressure to build up in the tube. While-the temperature of the apparatus was maintained at about 20 C., the increase of gas volume at one atmosphere was determined from readings of a manometer connected to the ozonizer tube.

The effect of the addition compounds at various concentrations in reducing gas evolution from the cable 011 under electrical stress is shown by the following tabulated data:

Results of these experiments are summarized as which these addition compounds reduce the gas evolution is that the molecules of the dissolved alkyl hydroxy aryl sulfide tend to diffuse and concentrate at gas-liquid interfaces of the insulator where they absorb the principal force of the electronic bombardment. Investigations have shown that these compounds are changed with more difiiculty than the hydrocarbons by electric discharges and that the changes they undergo are beneficial rather than prejudicial to their capability of avoiding copper corrosion and sludging, and even beneficial to their oxidation inhibiting action. Disulfides of this class of compounds undergo a marked improvement under the influence of electric discharges. The stability of insulating oils containing these compounds is also favored by the fact that these compounds have suitably low vapor pressures. The change which occurs in the addition compound may involve some polymerization and hydrogenation. In any event, this function of counteracting the deteriorating influence of gaseous electric discharges on insulating oils is obviously different from catalytic oxidation inhibition in which an addition compound is used in minimal amounts of no more than about 0.2% to effect a maximum degree of oxidation retardation.

In general, the amount of the preferred addition compounds to be used in reducing gas evolution by insulating oils under electrical stress is .within the limits of 1% to 10% by weight. The exact concentration depends upon characteristics of the oil or hydrocarbon dielectric and electri:

' cal apparatus to which the insulation is applied showing that an alkyl phenol sulfide does not appreciably reduce the gas formation unless it is present in the concentration more than 1% and that the soluble alkyl naphthoi sulfide is nearly twice as effective as the soluble alkyl phenol sulfide.

A possible explanation of the mechanism by as well as the particular compound added. As shown by the tabulated data, the compounds containing polynuclear aryl groups are very effective at concentrations below 1%, so that about .5% to 5% of such compounds is regarded as sufficient. The highly soluble alkyl phenol sul-, fides may be employed in concentrations as high as 10%.

Insulating oils to be improved in their stability, according to this invention, are preferably normally stable viscous mineral oils obtained by commercial refining treatments which eliminate unstable unsaturated hydrocarbon and non-hydrocarbon components. These treatments include sulfuric acid and clay refining, aluminum chloride refining, hydrogenation, and selective solvent processes. The resulting oil, if properly refined, is composed principally of highly saturated hyrocarbons, of parafilnic and naphthenic character, and may have a light color varying from pale yellow to white. The viscosity of the oil isappropriate for oil filled cables and transformers, if it is within the range of about 50 to 200 Saybolt Universal seconds at F. 'Oils 'having higher viscosities and even hydrocarbon wax or normally solid bitumens are used in solid filled cables. Light colored mineral oils havin viscosities of about 100 Saybolt seconds at 100 F. are normally used in oil filled cables. .Satisfactory hydrocarbon oils are obtained principally from parafilnic base and naphthenic base crude oils, since these oils with a practicable amount of refining have high dielectricastrengths and.

high oxidation stability. Other modifying agents, such as synthetic hydrocarbon pour point depressants and high molecular weight isobutylene polymer viscosity modifiers may be, added with the gas suppressing addition compound herein disclosed, but other agents which may be added for inhibiting corrosion, oxidation, and

sludging, are in general not necessary, since the alkyl hydroxy aryl sulfides by themselves are efiective against these deteriorating influences.

This invention is not to be limited by any theoretical explanation or examples presented herein by way of illustration. It is susceptible of obvious modifications which do not depart from the spirit of the invention.

We claim:

1. An electrical conductor insulated inside a casing with an organic insulation comprising a refined hydrocarbon oil of normally high chemical stability and a sufiilcient quantity of an alkyl hydroxy aryl mono-sulfide non-corrosive to copper miscible therewith to materially reduce gas evolution coming from the insulation due to the influence of high voltage electrical discharges on the oil.

2. An electrical conductor insulated in the manner described in claim 1 in which said alkyl hydroxy aryl sulfide is an alkyl phenol monosulfide in which the alkyl substituent contains 4 to 6 carbon atoms.

3. An electrical conductor insulated as described in claim 1 in which the alkyl group substituent of said alkyl hydroxy aryl mono-sulfide is selected from the class of secondary and tertiary butyl and amyl hydrocarbon radicals.

4. An electrical cable comprising an electrical conductor and a high dielectric strength insulation therefor spaced within a metallic casing, said insulation comprising a refined viscous mineral oil of normally high chemical stability and 1% to 10% of di(tertiary amyl phenol) mono-sulfide.

5. An insulated electrical conductor, said electrical conductor being surrounded within a casing by an insulation comprising a transformer oil and .5% to 5% of di(tertiary amyl naphthol) mono-sulfide.

6. An insulated electrical conductor comprising a casing enclosing a metallic electrical conductor wrapped with an insulating fabric impregnated with a refined hydrocarbon oil of normally high chemical stability containing a suilicient quantity of dissolved alkyl hydroxy aryl mono-sulfide to materially suppress gas evolution coming from the hydrocarbons due to the influence of a high voltage electrical discharge on the oil.

7. The method of reducing hydrogen gas evolution from a highly refined hydrocarbon insulating oil subjected to high voltage electrical discharges which decompose said oil, which comprises maintaining a sufiicient concentration in the range of 1% to 10% of an alkyl hydroxy aryl mono-sulfide dissolved in said oil to substantially reduce the gas evolved from the oil while the high-voltage electrical discharges are of sumcient intensity to cause decomposition of said oil and thereby liberate hydrogen from said oil.

8. The method as described in claim 7, in which the mono-sulfide is di(tertiary amyl phenol) mono-sulfide and the concentration of said mono-sulfide dissolved in said oil is from 5% to 10%.

9. The method as described in claim 7, in which the mono-sulfide is di(tertiary amyl naphthol) mono-sulfide and the concentration of said mono-sulfide dissolved in the oil is from 1% to 5%.

10. The method of insulating electrical conductors in electric cables carrying high voltages, w ich comprises placing and maintaining around s id electric conductors a refined mineral oil of normally high chemical stability suitable for electrical insulating purposes blended with from 1% to 10% of a compound constituted of 2' aryl groups linked together through a single sulfur atom, each of said aryl groups containing an alkyl and a hydroxy nuclear substituent and each of said alkyl group substituents having 4 to 6 carbon atoms.

11. An electrical cable comprising a cable sheath enclosing an electrical conductor and a high dielectric strength insulation therefor, said insulation comprising refined viscous mineral oil of normally high chemical stability blended with from about 5% to 10% of di(tertiary amyl phen01) mono-sulfide to substantially inhibit decomposition of the oil by high voltage electrical discharges in said cable.

WARREN M. SMITH. CARROLL J. WILSON. 

